Problem: In math class, the girl to boy ratio is $8$ to $5$. If there are $25$ boys, how many girls are there?
A ratio of $8$ girls to $5$ boys means that there is a group of $8$ girls for every group of $5$ boys. If there are $25$ boys, then there are $5$ groups of $5$ boys each. Because we know the ratio is $8:5$ , there must be $5$ groups of $8$ girls each. There is a total of $40$ girls in math class.